Thin membrane ionization pump apparatus

ABSTRACT

An ionization process vacuum pump. Ions produced in the pump are accelerated to energies sufficiently high to enable them to pass through a thin membrane and thus out of the system being evacuated. The membrane prevents return of low energy gas molecules from the high-pressure side of the pump. Embodiments relating to different methods for production of energetic ions are discussed.

United States Patent Thomas W. Snouse 13781 Pierce Road, Saratoga,Calif. 95070 849,607

Aug. 8, 1969 Aug. 24, 1971 Inventor Appl. No. Filed Patented THINMEMBRANE IONIZATION PUMP APPARATUS 3 Claims, 3 Drawing Figs.

U.S. Cl 417/48 Int. Cl r F04b 37/02 Field of Search 103/1 E; 230/69;60/202; 417/48, 49, 58

[56] References Cited UNITED STATES PATENTS 3,221,197 11/1965 Coppola230/69 X 3,328,960 7/1967 Martin 60/202 3,427,978 2/1969 Hanneman et a]103/1 Primary ExaminerRobert M. Walker PA TENTED AUG24 I97! 3,601, 503

FIGURE I FIGURE 2 THIN MEMBRANE IONIZATION PUMP APPARATUS The presentinvention is related to ion pumping for the production of low'pressuresand more specifically, to a new method for removal of the pumped gasesfrom the volume being evacuated.

Vacuum pumps 'utilizing ionization processes leading to a pumping actionhave been known for many years. In general, they utilized the processesof gettering active gases (fresh surfaces of the getter material may beprovided in several ways such as sublimation or by sputtering by thepositive ions which are to be pumped); and of ion burial (ions aredriven into the cathode and later buried by subsequent deposits ofmaterial sputtered from another cathode or sublimed from a gettersource). The distinguishing characteristic of said pumps being theremanence of the gases pumped within the body of the pump.

The aforementioned pump suffer from several drawbacks due to thischaracteristic. One drawback is the re-emission of previously pumped.gas due to sputtering. This contributes to the decline of pumping speedsof sputter-ion pumps at pressures of l -l0 torr. This factor may alsolimit the ultimate vacua attainable with some types of pump.

Another drawback is the poor tolerance of some pumps for hydrogen.Although it is pumped well initially, hydrogen eventually diffuses intothe getter material causing cracking,

spalling, and flaking which adversely affects the pump performance. I

Those pumps which form a getter film by sublimation have theinconvenience of requiring a cooled surface. Also, they may contributeunwanted gas molecules to the system as the getter material is sublimed.7

Some pumps, in order to provide surfaces for the deposit of gettermaterialhave multicelled anodes. This causes uneven distribution of theion current to the cathode which results in waste of cathode materialand shortened operating life. The size of the individual anode cell alsoaffects pump speed as a function of pressure and contributes to theaforementioned decline or pump speed at low pressures.

A further drawback is the poor ability of these pumps to pump noblegases; speeds for thesegases range from O to 24 percent of the speedsfor air.

It is the object of the present invention to provide a newionization-type vacuum pump which removes the pumped gases from thevolume being evacuated.

Briefly stated, in accordance with one teaching of the presentinvention, there is disclosed a diode ionization pump which has a thinmembrane as a cathode, thus allowing ions which are accelerated towardthe cathode to pass through it and into a chamber where they are removedby an auxiliary vacuum pump of any of several typescommerciallyavailable.

It is another object of this invention to employ a thin membrane as acathode in other types of ionization pumps to increase their pumpingspeed at low pressures and to alleviate other of their drawbacks asheretofore mentioned.

It is another object of this invention to provide a design for thelocation of an electron source in a thin membrane diode ionization pumpto facilitate the ionization process.

It is another object of this invention to employ a highly transparentanode together with a magnetic field parallel to the impressed electricfield as a means for increasing the path length of the electrons.

The features of this invention include a lower ultimate pres sure thanother ionization pumps, higher speeds at low pressures, ability to pumphydrogen and noble gases for sustained periods of time withoutsignificant re-emission, freedom from pump produced contaminants ordeposits, and the lack of a requirement for cooling.

.These and other objects and features of the present invention will beapparent to those skilled in the art by reference to the description ofthe drawings that follows, and in which:

FIG. I is a diagrammatic view of one embodiment of the presentinvention;

FIG. 2 is a diagrammatic view of a modification of the invention byinclusion of an electron source;

FIG. 3 is a diagrammatic view of another embodiment of the presentinvention.

Referring to FIG. 1 there is disclosed a vacuum pump incorporating thenovel features of the present invention comprising: an evacuableenvelope 11 adapted to contain gas molecules; a transparent anode 12 asa means to transfer energy to the ions and electrons participating inthe ionization process, and ameans 13 for allowing energetic ions topass from the discharge region 14 of the pump.

Envelope 1 l is rectangular both from the front and side and may be ofany dimension consistent with the considerations of good vacuumconductance and of economy in the production of magnetic field in thedischarge region 14. Envelope 11 is made of gas impervious material, forexample, stainless steel. At the top it is apertured for receiving, invacuumtight manner, a high-voltage feedthrough 15 which is in turnconnected to the anode 12. The envelope 11 is adapted tobe connected invacuumtight manner to a structure to be evacuated (not shown) throughthe intermediary of a high conductance passage 16 and a vacuum flange l7welded thereto, and is also adapted to be connected in the same mannerby means of conductance 18 and flange 19 to a backing vacuum pump (notshown). The working pressure of the backing pump is typically 10 to 10decades higher than that of the membrane pump; commercial ion pumps ordiffusion pumps would be suitable.

The cathode 13 is composed of any of several materials known to have alow vapor pressure at elevated temperature in vacuum and capable ofbeing formed into self-supporting hole-free membranes of thicknesses400'Angstrom' units (1.6 microinches) or less. Materials suitable forthis are suggested to be, but not limited to, carbon or aluminum oxide,with aluminum oxide as the preferred choice. It is suggested that thealuminum oxide be formed on an integral grid of aluminum for strengthand rigidity. This is done as follows. An aluminum member is shaped intoa rectangular can with'an open top and then etched to form a multitudeof small (about 3/16 inch square), thin (about 0.001 inch thick) areason the sides 20 and bottom 21. These areas are closely and regularlyplaced to leave a gridwork of thicker aluminum as a supportingstructure. The thinned areas are then treated by any of several methodsfamiliar to those skilled in the art, e.'g., according to that of'L.Harris, J. Opt. Soc. Am; 45,27 (1955 to form an aluminum oxide membrane.The cathode 13 is joined electrically and physically in a vacuumtightmanner to the envelope 11 near the top. This seal may beeffected in anyof several ways, as for example, by means'of an expansible inside collar(not shown). The envelope 11 (and by extension, the cathode 13) isconnected to ground: A magnet or magnets 22 providing a field strengthof 1,000 gauss, more or less, are placed flush with the outside walls ofthe envelope 1 1.

The anode 12 is a single cell and is constructed of very fine wiresstrung at large spacing over a supporting framework. It is made asnearly transparent as possible to afford the longest possible electronpaths. It is hollow for the same purpose and for the purpose of placingthe maximum feasible volume between the cathode membranes 13 at a highpotential such that the majority of the ions are formed inthis region.

The pump is operated in the following manner. It and the system to beevacuated are pumped to an intermediate pressure, typically 10" to l0torr. by'means of other vacuum pumps such as suitable trapped mechanicaland diffusion pumps, or by sorbtion and sputter ion pumps. The pump isbypassed (not shown) during pump down to avoid mechanical stress on themembrane-by the significant force differentials.

found at higher pressures. When the desired forevacuum is reached thebypass is closed and a high voltage, typically on the order of apositive potential of 15 to 20 kv. DC and produced by an external powersupply (not shown), is applied to the anode 12 by means of the feedthrough 15. This voltage initiates a Penning-type discharge one resultof which is the acceleration of ions toward the cathode. Most of theseions possess energies sufficient to enable them to pass completelythrough the membrane (A. Van Wijngaarden, and H. E. Duckworth, Can. J.Phys., 40,1749 (l962),.T. W. Snouse, J. Appl. Phys. Lttrs. 5,122,(1964). Thus the ions are completely removed from the pumping volume andare later disposed of by the backing vacuum pump. Gas molecules existingin the backing pump, on the other hand, are prevented from entering themembrane pump because their energy is insufficient to allow their directpassage through the membrane. Thus a net pumping action results,extending to very low pressures. The ultimate pressure will be limitedby extinction of the discharge, or, if the backing vacuum is not good,by the slow diffusion of gases back through the membrane. This pumpingmechanism is particularly suited to the lighter gases such as hydrogenand methane which are principal residual gases found in extreme highvacuum systems. It is not well suited to gases heavier than carbondioxide and sources of such gases such as elastomeric seals and vacuumoils and greases should not be employed. Additionally, both sides of thecathode membrane are subject to erosion by the bombarding ions. Sincethe number of said ions is directly proportional to pressure, it isprudent, in terms of long membrane life, to operate the pump at thelowest pressures possible where membrane lifetimes on the order of ayear in continuous operation are to be expected. Aside from thisconsideration, there is no reason why the pump could not be operated atmuch higher pressures.

FIG. 2 discloses an alternate embodiment of the present invention. Afilament 27 or other electron source is connected by means of avacuumtight feed through 28 to an external power supply (not shown)which is biased 500 to 2,000 volts negatively with respect to the anode12. The addition of relatively low energy electrons with properlydirected initial velocities from this electron source contributes to theionization efficiency in the anode region. Therefore, this'embodimenthas the advantage of easier starting at low pressures, and of increasedpumping speed due to increased ionization in the anode region.

FIG. 3 disclosed still another embodiment of the present inventionsimilar to the embodiment of FIG. 1 but modified in that the longelectron path lengths necessary to maintenance of an electricaldischarge at low pressure are obtained by confining the electrons in anelectrostatic field rather than in the mixed field of the priordescription. This is a modification of the orbitron pump (Herb, et al.,U.S. Pat. No. 3,244,969) or of the electrostatic getter ion pump of D.G. Bills et al., (U.S.

Pat. No. 3,407,991 No getter material is used. The grounded envelope 11is cylindrical, vacuumtight, with two suitable flanged vacuum apertures,17 and 19. The anode 12 is axially located and maintained at a positive15 to 20 kv. DC by an extemal power supply (not shown) by means of thehigh-voltage feedthrough 15. The cathode 13 is cylindrical but otherwisesimilar to that of FIG. 1, and is physically, in a vacuumtight manner,and electrically attached to the envelope 11 at points 33 and 34. Afilament 36, supplied with current by means of feedthrough 35, acts assource for electrons which orbit around anode 12. These electronsproduce ions which are accelerated by the electrostatic field, strikingand passing through the membrane as in the first embodiment. Thisembodiment has the advantage of eliminating the magnets whose bulk,cost, and attendant stray fields may be a negative factor in someapplications.

Having thus described the several useful and novel features of theembodiments of this invention, it will become immediately apparent thatthe several worthwhile objectives for which they were developed havebeen achieved. Although but a few specific embodiments of the inventionhave been illustrated and described herein, it is realized that otherconfigurations are likely to occur to those skilled in the art withinthe broad teaching thereof, hence, it is intended that the scope ofprotection afforded hereby shall be limited only insofar as saidlimitations are expressly set forth in the appended claims.

What is claimed is: l. A vacuum pump apparatus comprising:

a. an evacuable envelope adapted to contain gas molecules:

b. means for producing ionization in the pump, said means to consist ofa highly transparent anode maintained at high potential, together with acoparallel magnetic field;

c. means for passing ions thus produced out of the pump, said means toconsist of a very thin cathode together with a vacuumtight passage to anauxiliary pump.

2. The apparatus according to claim 1 wherein the ionization process isfacilitated by the addition of a suitably biased electron source betweenthe anode and cathode.

3. The apparatus according to claim 1 with these modifications:

a. a cylindrical vacuum envelope;

b. means for ionization, said means to consist of a central axiallylocated anode at high potential together with an electron source, suchconfiguration optimized to provide electron confinement in theelectrostatic field of the pumping volume;

c. means for passing ions out of the pump as in claim 1.

1. A vacuum pump apparatus comprising: a. an evacuable envelope adaptedto contain gas molecules: b. means for producing ionization in the pump,said means to consist of a highly transparent anode maintained at highpotential, together with a coparallel magnetic field; c. means forpassing ions thus produced out of the pump, said means to consist of avery thin cathode together with a vacuumtight passage to an auxiliarypump.
 2. The apparatus according to claim 1 wherein the ionizationprocess is facilitated by the addition of a suitably biased electronsource between the anode and cathode.
 3. The apparatus according toclaim 1 with these modifications: a. a cylindrical vacuum envelope; b.means for ionization, said means to consist of a central axially locatedanode at high potential together with an electron source, suchconfiguration optimized to provide electron confinement in theelectrostatic field of the pumping volume; c. means for passing ions outof the pump as in claim 1.